1. Field of the Invention
The invention relates to a metallic, iron-containing permanent mold with a coating, which can be exposed to a liquid or free-flowing aluminum material. In addition, the invention relates to a mold release agent for preparing a layer of this type, and a method for producing a layer of this type on a surface of a permanent mold.
2. Description of the Background Art
Due to the extremely high corrosion, which current metallic materials exhibit at typical processing temperatures in comparison with aluminum and other nonferrous metals, contact sites between the nonferrous metals and the permanent form must be treated with so-called mold release agents, so that failure-free operation is possible. Particularly but not exclusively in pressure casting processes, which are characterized by the use of high temperatures and pressures, highly different requirements, set forth hereinafter, are imposed on the mold release agents used thereby. Thus, the mold release agent must be used to support the metal flow, which leads to uniform filling of the permanent mold and, at the same time, the mold release agent is used to improve the final ductility of the cast parts. In addition, the mold release agent is used to avoid residues on the permanent mold, which can lead to imprecision in the mold. During the casting of the material in the permanent form, excess gas formation may not occur during the disintegration of the mold release agent, which would result in porosity of the molded parts. The mold release agent finally may also not contain any hazardous or toxic substances. The quality of the mold release agent is assessed depending on the meeting of these requirements.
A material known and used in mold release agents is boron nitrite (BN), which is similar to graphite in its crystal structure. Like graphite, it has a low wettability for many substances, such as, for example, silicate melts or metal melts. For this reason, there are many studies on nonadherent layers based on boron nitrite to utilize them for casting processes. The problem in the case of this utilization, however, is that permanent application of boron nitrate in bulk onto molds, particularly of a complex nature, does not succeed. A method for the permanent application of a temperature-stable, corrosion-resistant mold release layer is described in German Patent Application No. DE 198 42 660 A1. In this case, a boron nitrite powder is applied to the surface of a permanent mold using electrostatic coating.
Attempts were also made to produce binding agents on an inorganic base into which boron nitrite is integrated. The production of boron nitrite protective layers with thicknesses of 0.2 to 0.7 mm on refractory materials for the continuous casting of steel is described in U.S. Pat. No. 6,051,058. In this case, boron nitrite is bound to the refractory material on the order of 20 to 50% by weight with use of high temperature binders in the form of an aqueous coating solution based on metal oxides from the group including ZrO2, zirconium silicates, as Al2O3, SiO2, or aluminum phosphates.
To suppress wear and the corrosion of materials, a wear protection layer, into which functional materials are integrated in a binder matrix, is known from German Patent Application No. DE 101 24 434 A1, which corresponds to U.S. Publication No. 20020192511. This so-called functional coating having an inorganic matrix phase, including at least largely of a phosphate, and a functional material, which is embedded therein and may be, for example, a metal, graphite, a hard material, a dry lubricant, an aluminum oxide, a silicon carbide, etc. A method for producing this functional coating is also described, whereby a functional material in powder form is dissolved in a liquid component, which may be, for example, water, and combined with phosphoric acid to generate a phosphate. A matrix solution, made up in such a way, with the liquid component and the phosphate can also be called a gel because of its consistency. After a material is coated with this matrix solution, the material is subjected to heat treatment, so that an adherent functional coating forms on the base material.